I have a 25-30yr old forced hot air furnace and even thou it is running good I think I should replace it and add an A/C unit to it.

I have a few issues:
1. my house is a front to back split ranch with 4 levels and the top level is always the hottest throught the house. Will I be ok adding an A/C coil to the new furnace or will I be better with an air handler installed in the attic..?
top level has 3 bedrooms and a bath(access to the ceiling from the attic)
mid level has the kitchen, livingroom and dining room(access to the ceiling from the attic)
lower level has a family room, laundry room and another full bath
4th level is a full basement

2.Also the existing duct has 4" duct pipes providing heat. Is a 4" pipe big anough to push the colder denser cold air..?
will the house be better served with an air handler..?

3. the return duct work was formed from closing the gap between rafters...is that ok or should I invest in new return ductwork..?